It doesn't matter who the Reminder Card is in front of; it's just a reminder. If table configuration allows, it is probably most convenient to put the Reminder Card between the players who have used the Power Card so that they can all easily reference it.
For example, if two Spirits each use Bargains of Power and Protection once, both of those Spirits grant Defend 1 to each Dahan within Range 1; it doesn't matter which one more recently used the Power. If a single Dahan is within Range 1 of both Spirits, that Dahan will provide Defend 2, since the two Defend 1 effects stack.